1. Field of the Invention
The present invention relates to a recording and reproducing apparatus. More particularly, it relates to a recording and reproducing apparatus in which a holographic recording medium is irradiated with an information light beam and a reference light beam at the same time, and multiplex-recording of page data is performed in the same recording region in view of characteristics of the recording region.
2. Description of the Related Art
On a holographic recording medium, an information light beam corresponding to two-dimensional page data and a reference light beam are irradiated in the same region of the medium at the same time, thereby recording the two-dimensional page data in said region. Since the page data is recorded as interference fringes of the two light beams (information light beam and reference light beam), the plurality of page data can be multiplex-recorded in the same region of the medium.
In a method of multiplex-recording, an angular multiplex-recording method and a shift multiplex-recording method are suggested, for example. In an angular multiplex-recording method, an angle of the reference light beam applied to one recording region is varied. In a shift multiplex-recording method, multiplex-recordings are performed with a part of the region overlapped with each other while the recording region on the medium is slightly shifted, and the like.
In a conventional magnetic optical disc such as an MO or a DVD, the multiplex-recording in the same region could not be performed. But in the holographic recording method, multiplex-recording in the same region can be performed, and recording capacity can be considerably increased. For example, according to the angular multiplex-recording, in case that the recording region of the medium is divided into 50 (A1 to A50), an information light beam 10 and a reference light beam 11 are irradiated to each minimum recording unit region and multiplex-recordings of a plurality of page data are performed by changing an irradiation angle of the reference light beam as shown in FIG. 13.
FIG. 13 shows a case where the multiplex-recordings are performed by irradiating a recording unit region A2 with N reference light beams having different angles. In this case, N different page data can be multiplex-recorded in the same region A2 (refer to FIG. 14). The multiplicity (multiplex-recording number) in this case is N.
As one of documents disclosing a multiplex-recording method, there is known Japanese Unexamined Patent Publication No. 2002-216359.
However, it is difficult to manufacture the holographic recording medium so that all recording regions have completely uniform characteristics, since a recording material for recording data in the holographic recording medium is a photosensitive material. Thus, the local recording region of the medium has a large variation in recording characteristics in some cases.
For example, although 10 recorded page data can be normally reproduced in regions A1 and A2 in FIG. 13, when 10 recorded page data have been multiplex-recorded in lower right regions A48 and A49, it is often the case that a reproduction error occurs. In such case, only 7 page data can be multiplex-recorded at most in regions A48 and A49. Thus, there is a variation in recording characteristics.
As shown in FIG. 15, when the numbers of page data which can be multiplex-recorded differ in 6 recording unit regions within a range of M+1 to M+3, a value smaller than the number of page data which can be multiplex-recorded, that is, M is set to the maximum multiplex-recordable number of the medium, in view of reliability of recording and reproducing at the time of shipment.
It is considered that such variation in recording and reproducing characteristics is caused by a problem of a manufacturing process, a physical defect which could be generated in each layer which constitutes the medium, a variation in optical characteristics due to non-uniformity of a recording and reproducing apparatus or a medium material and the like.
However, even when the maximum multiplex-recording number M (maximum multiplicity) of the medium is ensured at the time of shipment, a recording unit region in which M (corresponding to the maximum multiplex-recording number M) page data cannot be multiplex-recorded could be generated due to a local variation in characteristics.
FIG. 16 shows a medium in which there is a larger variation in characteristics.
Referring to FIG. 16, the multiplex-recordable numbers in recording regions A1 to A6 are M, M+3, M−3, M+1, M and M−1, respectively.
In this case, although the M page was ensured as the maximum multiplex-recording number, multiplex-recordings of M−3 pages and multiplex-recordings of M−1 pages only can be performed in the recording regions A3 and A6 actually, respectively.
According to the recording unit region A3, although recordings can be normally performed until M−3 page data, when multiplex-recording is further performed, the page data (in M−2 and M−1) cannot be normally recorded and not only that, the first page data, for example could not be reproduced.
This is because multiplex-recording is performed by excessive exposure for the recording region having multiplex-recordable number actually smaller than the maximum recording number M beyond its recording performance, so that reproduction characteristics of the normally recorded page data could deteriorate. In other words, the page data cannot be recorded and already recorded page data could be destroyed.
Therefore, in order to ensure sufficient reliability of the recording and reproducing characteristics, lower maximum multiplicity L (<M) is set for all of the recording regions uniformly at the time of shipment as shown in FIG. 16. Alternatively, a strict test is performed before shipment and a medium having large variation in characteristics in which the maximum multiplicity M cannot be ensured may be determined as a defective medium so as not to be shipped. However, in this case, a yield of the medium is lowered and its manufacturing cost becomes high.
Meanwhile, when the lower maximum multiplicity L is guaranteed in order to ensure the sufficient reliability, there is a case in which a recording region having very preferable characteristic exists actually. For example, like the region A2 in FIG. 16, a recording region in which M+3 (greater than L) page data can be multiplex-recorded could exist. Although more page data can be recorded in the region A2, since the maximum multiplicity is set to L, the recordable space cannot be effectively used.
Referring to FIG. 16, since the maximum multiplicity is set to L, multiplex-recording is only performed L times for all regions, so that the recording space corresponding to a hatched part lower than the recording number L cannot be effectively used.